Prior art overstress protection devices are functionally sensitive to the fluid pressure of the hydraulic system. In such systems, a load applied to the boom in certain positions, will cause a high pressure overload to be sensed within the hydraulic system, but the same load carried by the boom positioned in a different configuration may not cause such a pressure overload. Consequently, under such circumstances, the boom is often overloaded without causing a pressure overload within the hydraulic system. As a further consequence, booms are quite often permanently bent and seriously damaged. For example, the same load will cause much greater pressure within the hydraulic cylinders when the boom is in the completely extended position, than when the boom is positioned in a configuration bending acutely at its elbow joint. Thus, the boom while at an acute angle may be seriously overloaded by the operator without the overload sensors detecting same, thereby causing serious damage to the boom.
Moreover, if a lower pressure limit is selected to compensate for the possible overloading of the boom while configured in such acute angle positions, then the utility of the hydraulic derrick-type equipment is seriously limited with respect to use of the same when the arm is at its full extension, since only small loads could be safely carried, thereby defeating a major purpose of the equipment. Thus, by using my improved overload protection apparatus, this problem is effectively eliminated since my invention senses overload conditions as a function of the extent of boom flex.